At present, the continuous-variable quantum key distribution (CV-QKD) protocol based on discrete modulation has received more attention. In this paper, we propose a three-state CV-QKD protocol based on post-selection. The security key rate of the proposed protocol is calculated under collective attack and reverse reconciliation and compared with that of the four-state protocol. The simulation results show that the proposed three-state protocol can outperform the four-state protocol in security rate if the transmission distance is not too long, indicating that the three-state protocol is more feasible for a short-range application, such as metropolitan area networks.